It's as effective as the pill. But you're right that it doesn't provide birth control during the fertile days.

It depends on how you define birth control. Is it something that enables you to have sex during fertile days or is it something that enables you to know more about your fertility?

So, seen in the first way, the sympto-thermal method is just a method for observing natural processes. Nothing that prevents pregnancy during the fertile window.

If you want mechanical birth control, there are really only three reversible options: condoms, IUD, diaphragm. Everything else that's reliable is based on hormones. In that light, I think it's a good idea to use the sympto-thermal method to be at least 50% less dependant on other mechanical tools.